A polymer electrolytic cell, such as a rechargeable lithium ion battery, is commonly constructed by means of the lamination of electrode and separator cell elements which are individually prepared. Each of the electrodes and the electrolyte film/separator is formed individually, for example by coating, extrusion, or otherwise, from compositions including binder materials and a plasticizer.
In the construction of a standard lithium-ion cell, for example, an anodic current collector may be positioned adjacent an anode (negative electrode) film, or sandwiched between two separate anode films, to form the negative electrode.
Similarly, a cathodic current collector may be positioned adjacent a cathode (positive electrode) film, or sandwiched between two separate cathode films, to form the positive electrode.
A separator is positioned between the negative electrode and the a positive electrode. The anode, separator, and cathode structures are then laminated to produce a unitary flexible electrolytic cell precursor structure.
An extraction process is used to prepare the cell precursor for activation with electrolyte. During processing of the cell precursor, a large quantity of a homogeneously distributed plasticizer is present in the solid polymeric matrix. Prior to activation of the electrolytic cell or battery, however, the organic solvent is removed. This is generally accomplished using an extracting solvent such as diethyl ether or hexane, or the application of a vacuum, which selectively extracts the plasticizer without significantly affecting the polymer matrix. This produces a "dry" electrolytic cell precursor, which does not include any electrolyte solvent or salt. An electrolyte solvent and electrolyte salt solution is imbibed into the "dry" electrolytic cell copolymer membrane structure to yield a functional electrolytic cell system.
A lithium ion battery typically comprises several solid, secondary electrolytic cells in which the current from each of the cells is accumulated by a conventional current collector, so that the total current generated by the battery is roughly the sum of the current generated from each of the individual electrolytic cells employed in the battery. In lithium ion batteries it is common to stack separate electrolyte cells to create the battery.
Lithium ion electrolytic cells 101 may be of the traditional "sandwich" type, shown in FIG. 1, with a cathode 110, a separator 112, and an anode 114 sandwiched together. However, there is a trend to develop "bi-cells", in which two anodes or two cathodes are present, surrounding a single opposite electrode. A cathode-out bi-cell 201, such as that shown in FIG. 2, includes, sequentially, a first cathodic electrode 210a, a first separator 212a, a central anode 214, a second separator 212b, and a second cathodic electrode 210b. Electrode tabs connect the anode and cathode elements to the exterior of the package. Batteries constructed of cathode-out bi-cells tend to successfully pass abuse tests which are geared toward crushing, but do not pass abuse tests in which a nail is driven through the battery.
An anode-out bi-cell 301, for example that shown in FIG. 3, includes, sequentially, a first anodic electrode 314a, a first separator 312a, a central cathode 310, a second separator 312b, and a second anodic electrode 314b. Electrode tabs connect the anode and cathode elements to the exterior of the package.
Batteries constructed of anode-out bi-cells tend to successfully pass abuse tests in which a nail is driven through a battery, but commonly fail crush abuse tests. A battery consisting of two anode-out bi-cells 401a, 401b, is shown in FIG. 4. Batteries such as that shown in FIG. 4 commonly include two to six anode-out bi-cells, and may include more bi-cells.
One approach to limiting the failure of bi-cell batteries during abuse testing has been the limitation of the total battery capacity, which limits the possibility of adverse affects during abuse testing.
A parallel approach to limiting the failure effects of bi-cell batteries has been a limitation on the use of high surface area graphite within the electrodes. The use of such graphite has been limited in the prior art due to the undesirable reactivity of batteries under conditions of abuse. The industry response has been to forgo the use of high surface area graphite and the increased potential it provides in order to provide safer batteries.
In view of the above shortcomings associated with the prior art, there is a need for solid state electrochemical devices that are safer than those previously disclosed, and which can safely provide batteries having higher energy density and increased potential than previously available.